The present invention relates to the art of angiographic examinations. It finds particular application in conjunction with angiographic examinations of a patient's abdomen and lower extremities with x-rays in which the x-ray source/detector and the patient move relative to each other and will be described with particular reference thereto. However, it is to be appreciated that the present invention will find application in other angiographic examinations or examinations of other moving substances.
Heretofore, digital x-ray systems have been used for angiographic examinations. An x-ray source was disposed on one side of the patient and an x-ray detector disposed on the other side. The x-ray detector converted x-rays which had passed through the patient into visible light which was converted to a digital video signal. Because blood is relatively transparent to x-rays, the patient was injected with a radiopaque dye which had relatively good x-ray absorption such that blood vessels showed up dark in the resultant image. Images of the circulatory system only were made by subtracting a processed reference or basis image taken before injection of the dye from the processed image taken after injection of the dye.
One application of x-ray angiography is imaging blood flow in a patient's lower extremities. The radiopaque dye is introduced into an artery in the pelvic area and flows with the blood through the patient's leg. In a normal healthy patient with good circulation, the dye moves from the pelvic area to the toes quickly, perhaps in 15 seconds. However, in a patient with arterial blockage, typical candidates for such a procedure, the radiopaque dye may require a minute or so to traverse the same course. The travel duration is relatively predictable from the preliminary diagnosis of the patient's condition.
In the prior systems, the x-ray source/detector and patient were moved relative to each other and a series of images were taken along the length of the leg at perhaps five or six positions. Ideally, the images at each position or station were collected as the peak of the radiopaque dye passed through the center of the imaged region. In this manner, images with peak blood opacity were sought at each position. One of the difficulties was gauging when to collect the image without over-irradiating the subject and any nearby medical personnel.
One technique for limiting x-ray exposure while assuring meaningful images was to preprogram the x-ray exposure rates. To assure that an image was collected near the maximum opacity in fast moving blood regions such as near the pelvis, a relatively high frame or image rate was needed, e.g., 3-4 frames/second. As the blood flow slows, subsequent positions have lower frame rates, typically down to about 0.5 frames/second, i.e. 1 frame every 2 seconds. The frame rates used at each location were predetermined prior to the examination. These rates were estimates based on experience and expected pathology.
By reducing the number of frames in the areas of slower blood flow, radiation dosage is reduced. One disadvantage of using predetermined scanning programs is that unexpected blood flow can result in a failure to image when the arteries show maximum opacification. Images with partial opacification are of less diagnostic value than those with maximum opacification. If images are taken with substantially no radiopaque dye present, the procedure normally needs to be repeated with a different set of predetermined frame rates. Repeated injection of radiopaque dye is undesirable because of possible effects on the kidneys which must remove the dye from the blood.
A second disadvantage is that the predetermined frame rates are usually found to be too high, because they should be able to capture maximum opacification in the cases with highest expected flow rates. Patient dose could be reduced if frame rates were not set for the worst case.
The present invention provides a new and improved imaging method and system which overcomes the above problems and others.